Relays are used in telecommunications networks to not only forward signals between mobile terminals and the core network, but to add power to the signal. An equivalent term for a relay is a repeater. These are employed in various types of telecommunication systems, including the Long Term Evolution (LTE)/System Architecture Evolution (SAE) currently being developed by the “3rd Generation Partnership Project” (3GPP). In this regard, Long Term Evolution (LTE) is an advanced version of UMTS that uses E-UTRA (Evolved Universal Terrestrial Radio Access), and which employs OFDMA (Orthogonal Frequency Division Multiple Access) on the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) on the uplink.
The Radio Access Network component of the LTE/SAE is called the E-UTRAN, which comprises eNode Bs (eNBs). The eNBs provide both user plane and control plane (RRC) protocol terminations towards the mobile terminals (UEs) in the network. The eNBs are interconnected with each other by means of the X2 interface, and are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. These are the channels over which communications are made in the network.
According to 3GPP TS 36.300, there is a functional split between the different elements of the LTE network, which is illustrated in FIG. 1. In particular, it is to be noted that eNBs control the dynamic allocation of resources (i.e. scheduling) to the UE in both the uplink and downlink.
Transmissions over such wireless uplink and downlink channels are subject to errors, for example due to receiver noise and unpredictable interference variations. Therefore, virtually all wireless communications systems employ some form of Forward Error Correction (FEC). The basic principle of forward error-correcting coding is to introduce redundancy in the transmitted signal. This is achieved by adding parity bits to the information bits prior to transmission (alternatively, the transmission could consists of parity bits alone, depending on the coding scheme used). In a further variation the parity bits may be “punctured” by removing some of the parity bits after encoding with an error correction code. The parity bits are computed from the information bits using a method given by the coding structure used. Thus, the number of bits transmitted over the channel is larger than the number of original information bits and a certain amount of redundancy has been introduced in the transmitted signal.
Another approach to handle transmissions errors is to use Automatic Repeat Request (ARQ). In an ARQ scheme, the receiver uses an error-detecting code, typically a Cyclic Redundancy Check (CRC), to detect if the received packet is in error or not. If no error is detected in the received data packet, the received data is declared error-free and the transmitter is notified by sending a positive acknowledgment (ACK). On the other hand, if an error is detected, the receiver discards the received data and notifies the transmitter via a return channel by sending a negative acknowledgment (NAK). In response to a NAK, the transmitter retransmits the same information.
Hybrid ARQ (HARQ) is a combination of forward error-correcting coding and ARQ. HARQ uses forward error correcting codes to correct a subset of all errors and relies on error detection to detect uncorrectable errors. Erroneously received packets are discarded and the receiver requests retransmissions of corrupted packets.
Whilst these techniques do improve the transmission efficiency, there is still room for improvement. This is particularly the case in modern mobile communications networks, where users are demanding higher data rates. At present, typically only those mobile terminals that are in close proximity to a base station (eNB) can achieve a high data rate, as interference affects the data rate as the distance between the base station and the user terminal increases.
A further problem for LTE networks is that since LTE needs to be compatible with both LTE compatible terminals and legacy terminals (such as Rel-8 terminals to which relays are transparent), there is a challenge to integrate relays in such a network environment without unduly increasing network signalling.
The use of relay nodes has been proposed to distribute the data rate more evenly in a cell served by a particular base station. This approach uses one or more relays for a single transmission. Whilst such a relay system can greatly increase the data throughput, an inherent problem with relay nodes (RN) is that in most situations, a given node cannot simultaneously transmit and receive at the same frequency band, due to the transmitting and receiving antennas not being well separated. Therefore each RN is not able to listen while transmitting, and vice versa, which introduces restrictions on their operation. There are now two types of relays defined in LTE A study: Type I relay has its own cell ID, and therefore deployed as a separate cell while using donor cell resources for backhauling; Type II relay doesn't have its own ID, and will therefore not introduce any new cell perceived by a UE.
The L2 transparent relay has been studied in 3GPP RAN1 as a candidate technology for LTE Advanced. LTE-Advanced extends LTE Rel-8 with, inter alia, support for relaying as a tool to improve the coverage of high data rates, group mobility, the cell-edge throughput and/or to provide coverage in new areas. In this regard, the discussions have been in relation to potentially introducing a type of transparent L2 relay. The relay is ideally transparent in order to be backward compatible with Rel-8 UEs.
In the context of the proposed type II transparent relay, the RN will be properly located so that the eNB-RN link is of good quality and preferably also to improve an area not well covered by the donor eNB. The RN monitors the signalling exchange between the donor eNB and the targeted UEs, so that the RN is aware of scheduling information and HARQ acknowledgement information exchanged between a UE and the donor eNB. The RN's good geometry in the donor eNB cell enables it to decode at the early phase of a HARQ process for targeted relay UEs. The RN will then be able to contribute in later HARQ transmissions by synchronized retransmission for the UE with the eNB. In this way, the RN effectively increases the signal strength transmitted between eNB and UE. As a transparent relay (i.e. to the UE), the RN does not send Cell specific Reference Signal (CRS) and therefore does not have any physical cell identity. In other word, the RN behaves like a 3rd party “agent” intercepting communications between UE and eNB, and trying to help the communication by participating in the HARQ retransmissions in uplink or downlink or both.
In the context of this type of relay, in the downlink a relay UE will see, in a sub-frame where HARQ retransmission occurs, the control symbols from eNB and PDSCH (Physical Downlink Shared Channel) symbols from RN or from both RN and eNB. Accordingly, the time-frequency resources need to be synchronized for the RN, donor eNB and UE (i.e. the transmission time and frequency carriers need to be the same at retransmission). Similarly such synchronization needs to be maintained in the uplink. A method of achieving synchronisation in retransmission for the RN with the eNB (in downlink) or with the UE (in uplink) by pre-scheduling for synchronous HARQ retransmission and the use of UE specific reference signal for demodulation is described in detail in European patent application 10155254.5.
Synchronous HARQ transmission means that once the first HARQ transmission is decided, later HARQ retransmissions will follow a pre-defined pattern in the time-frequency resource plane. That means, to schedule such synchronous HARQ transmission, the eNB at (or before) the first transmission also effectively schedules the subsequent retransmissions for that HARQ process in pre-arranged resources obeying a common HARQ retransmission pattern known also by the RN, so that the RN, once having decoded the data at the early phase of the HARQ transmission, can pre-schedule exactly the same HARQ retransmission at the same time-frequency resources (i.e. identically to the schedule planned at eNB).
A TDM (Time Division Mode) constraint applies to this type of transparent relay, because the RN should not listen and transmit at the same time for a given time-frequency resource due to difficulty in antenna isolation for the transmitting and receiving directions at the RN. This TDM constraint therefore requires that the RN be in receiving mode in the Relay sub-frame where the first HARQ transmission(s) is/are scheduled, and to be in transmitting mode in the sub-frame where retransmission is scheduled the RN. To meet such TDM constraints, sub-frame patterns should be designed so that in a defined set of sub-frames where the RN is listening (i.e. in the RN Receiving Sub-Frame (R-R-SF) set), and in a set of subsequent sub-frames where the RN is transmitting (i.e. the RN Transmitting Sub-Frames (R-T-SF) set) the transmitting and receiving sub-frames occur in a fixed pattern, typically with a period corresponding to the HARQ retransmission interval (i.e. typically greater than Round Trip Time). For the TDM constraint, the R-R-SF set shall not overlap with the R-T-SF set.
The design of sub frame patterns for R-R-SF and R-T-SF depend upon planning issues such as how much resources can be used for relay assisted traffic. In particular, it has been advantageously determined that valid sub-frame patterns depend on both the set of sub-frames selected for R-R-SF and the maximum number of HARQ transmission (M) for RFT.
The use of relays can undesirably cause communication resource waste due to the potential for relay nodes to interfere with each other. It is therefore desirable to devise a means for reducing this resource waste and improve data transmission rates in mobile networks.